The knee comprises three inter-dependent joints in three separate compartments, all surrounded by a fibrous capsule covered by the skin. The medial tibio-femoral joint involves contact between the thigh bone (the femur) and the leg bone (the tibia) on the inside of the lower limb. The lateral tibio-femoral joint involves contact between the femur and the tibia on the outside of the lower limb. The patello-femoral joint involves contact between the femur and the knee cap (the patella) on the front of the lower limb.
The front of the lower (distal) end of the femur comprises an anticlastic flanged groove, convex in the sagittal plane, transversely concave, providing a track for the patella. The back of the distal femur divides into two separate near-spherical convex condyles making contact with the tibia. The upper surface of the tibia is like a plateau which is slightly dished on the medial side for contact with the medial femoral condyle forming the medial tibio-femoral joint and slightly convex on the lateral side for contact with the lateral femoral condyle forming the lateral tibio-femoral joint with a protrusion (the tibial eminence) running from front to back between the joints.
The articulating surfaces in each joint are covered with thin layers of a tough protective layer, called cartilage, and are lubricated by synovial fluid secreted from a membrane on the inner surface of the fibrous capsule surrounding the knee. The surfaces of the tibio-femoral joints are further separated by the menisci, semi-circular semi-lunar collagen bundles oriented circumferentially. Each bundle is securely attached at each end to the tibia and loosely to the peripheral capsule. The menisci form closely-fitting mobile sockets for the femoral condyles bringing the dissimilar surfaces of the femur and tibia into closer conformity while allowing some antero-posterior translation of the femoral condyles on the tibia.
The bones are held together actively by muscles with their tendons which span the joints and passively by ligaments and the joint capsule. The ligaments comprise bundles of collagen fibres running mainly longitudinally. The collateral ligaments arise on the external surfaces of the medial and lateral condyles. The medial collateral ligament inserts into the external medial surface of the proximal tibia. The lateral collateral ligament inserts into the proximal surface of the fibula. The medial collateral ligament is a much larger and stiffer structure than the lateral collateral ligament. The cruciate ligaments arise from the internal surfaces of the femoral condyles and insert into the tibial eminence.
The ligaments and the bones together form a mechanism which controls a complex pattern of movement of the bones on each other. In the unloaded state, flexion of the knee to 130° about a transverse axis is accompanied by approximately 25° rotation about the axis of the tibia (axial rotation). These movements are accommodated by mainly antero-posterior translations of the tibio-femoral contact areas so that the bones roll as well as slide on each other and the patella slides over the anterior femur. Additionally, the femoral condyles can spin about the axis f the tibia. The joint also allows approximately 5° of rotation about an anteroposterior axis (abduction-adduction). Under load, the ligaments stretch and the articular surfaces indent, significantly modifying the relationship between flexion, axial rotation and abduction-adduction and between flexion and contact area translations. Movements at the knee are therefore load- and activity-dependent.
Damage to the articular surfaces or to the ligaments changes the patterns of movement of the bones on each other and the response of the joint to load. Osteoarthritis follows from failure of the cartilage in one or other of the three joints, leading to bone-on-bone contact and the onset of pain. Frequently, osteoarthritis first manifests itself in the medial compartment, while the ligaments remain intact. The disease can remain confined to the medial compartment until the anterior cruciate ligament fails and the disease then spreads to the other two compartments. No drug based treatment has been found which halts or reverses these processes.
Total knee replacement is the most common surgical treatment for osteoarthritis, involving replacement of the articular surfaces of all three compartments and sacrifice of some of the ligaments. Partial knee replacement involves replacement of the articular surfaces in only one compartment, leaving intact the surfaces of the other two compartments and all of the ligaments. Partial knee replacement can act prophylactically, reducing the rate of development of the disease in the other compartments. Partial knee replacement is surgically more demanding and is therefore not always used when it is indicated.
To implant the prosthetic components of a knee replacement, sufficient sections of bone have to be removed from the surfaces of the tibia and the femur. The component parts of the prosthesis are then fitted accurately replacing the material removed by the surgeon.
Mobile bearing arthroplasty uses metal components fixed to the tibia and the femur with an intervening plastic bearing, an analogue of the natural meniscus, interposed therebetween. The bearing provides a mobile socket to bring the femoral component into conformity with the tibial component. The bearing has a concave socket on its upper surface for contact with the femoral component and a flatter lower surface for contact with the tibial component. The metal components are fixed to the bones so as to leave a constant minimum gap between them when the knee is flexed and extended. The most appropriate thickness of bearing is then chosen to fill that gap.
On implantation, the bearing is pushed between the metal femoral and tibial components against the resistance of stretching ligaments. This requires the thickest part of the posterior portion of the bearing to fit through the minimum gap between the round femoral component and the flatter tibial component. The ligament stretch required is the difference between the maximum thickness of the posterior end of the bearing and the minimum thickness of the bearing. This difference is known as the entrapment of the bearing.
In one version of the prior art, there are five sizes of bearing, from extra small to extra large, with dimensions other than thickness which vary parametrically with the radius of the spherical surface of the femoral component. The entrapment varies from about 3 mm in the extra-small to about 4 mm in the extra-large. The range of sizes is necessary to fit both small patients and large patients.
A complication of mobile bearing arthroplasty can be slack ligaments. As a result, the surgeon naturally seeks to use the thickest possible bearing to avoid slackness. Care has to be taken not to overstuff the joint, whereby a bearing that is too thick is selected. This leads to pain and failure of the components and, in the case of a partial knee replacement, degeneration of the preserved compartments.
It has been appreciated that post-operative pain is most commonly experienced after implantation of the smaller components. However, pain can also occur in larger bearings, as they require significant force for implantation, provided by the surgeon's thumbs.
It has been appreciated that some patients continue to have pain after a mobile bearing partial knee replacement. There is need for a bearing design with more entrapment for the smaller bearings and less entrapment for the larger bearings which may prevent overstuffing of the joint.